Optical discs have been widely spread as the optical information recording media with which recording or reproduction of information is performed by irradiation of laser beams. These types of optical discs include CD-ROMs and DVD-ROMs having embossed pit strings formed in advance to be used exclusively for reproduction, CD-Rs and DVD-Rs to which data can be recorded only once, CD-RWs, DVD-RWs, and DVD-RAMs which are capable of rewriting, etc. Each of these optical discs is structured to perform recording or reproduction of information by irradiating a converging beam spot to an information recording layer via a transparent substrate. Pit strings or grooves are formed on the information recording layer along a spiral recording track.
Among those optical discs, there is such a type that has an area called a burst cut area (BCA) as information that can be reproduced by a converging beam spot of an optical disc device, in addition to having the spiral recording track. This BCA is formed with segments extending in a radial direction by being arranged at prescribed intervals on a circumference in a low-reflectance form, and the pattern thereof is determined according to the contents of information to be recorded. In a sense, it is in a structure where a barcode used in general is provided circumferentially. Burst cut information such as the type (ROM, write-once type, or rewritable type) of the optical information recording medium and the copyright managed number is recorded in the BCA.
FIG. 4 shows an illustration of a BCA 101 and a fragmentary enlarged view thereof for explanations. As shown in FIG. 4, the BCA 101 is provided in a radius position range on a rotation center side (inside diameter side) of a specific recording layer of an optical information recording medium (optical disc) that includes a multilayered recording layer. The BCA 101 is formed with a plurality of low-reflectance segments 101 lined on a circumference in such a manner that the segments in a uniform length are lined in a radial direction as if short strips are lined at a small pitch to form a prescribed pattern.
The area where the BCA 101 is provided has no track for a converging beam spot “e” to follow. Thus, the optical disc device scans the converging beam spot “e” on the BCA 101 in the circumferential direction as indicated by an arrow S in accordance with rotation, while applying only the focus servo. At this time, the reflected light amount of the converging beam spot “e” is fluctuated depending on the patterns and it is outputted as a signal. Therefore, information associated with the BCA 101 recorded on the above-described optical disc can be reproduced based on the signal.
An example of such BCA 101 is depicted in a standardization material (ECMA-267; 120 mm DVD-Read-Only Disc) of ECMA (European Computer Manufacture Association).
Comparing the recording of information by using the BCA with the recording of information by using the pits, the information recording with the BCA is advantageous in respect that the BCA can be formed individually for each optical disc after manufacturing the optical discs. For example, the copyright management number can be added to the optical disc.
In this case, as a way to form the BCA in an optical disc such as a ROM used exclusively for reproduction, a metal reflection film formed on an information recording layer may be removed by irradiation of a strong laser thereby to form a low-reflectance part.
However, the interval of the pattern that can be formed with this method (the method for forming the BCA after manufacturing the optical disc) becomes considerably larger than the interval of the pits. For example, the practical value of the width of the low-reflectance part 101a is about 10 μm, and the widest used pattern interval is 100 μm or more.
Patent Document 1: International Patent Publication W2002/037483